A. Field of the Invention
The present invention relates to a new and improved method and apparatus for monitoring coagulation times. More particularly, the present invention is directed to an improved shear ultrasonic coagulation monitor for measuring prothrombin times and related coagulation times, and to an improved method for measuring the same using one or more piezoelectric crystals.
B. Description of the Background Art
Measurements of blood coagulation times are of clinical interest in evaluating patients with liver disease, and when clotting abnormalities are known to occur or are suspected. Patients on long term anticoagulant drugs require these measurements for the purpose of dosage adjustments. Generally, these measurements provide a rate-reaction record of clot formation which is characteristic of specific blood coagulation abnormalities. The instruments that perform these measurements allow recording of the entire clotting process in whole blood plasma and are suited to a wide range of clinical diagnostic applications and research studies.
Various tests and instruments for measuring clot formation are known. The simplest procedure for performing these tests is a manual test wherein a technician views a sample of blood and records the time required for a clot to form. Manual testing is simple but requires the total attention of the technician and quick reflexes, otherwise the test becomes error prone. Also, special lighting is required to permit the technician to see the clot formation.
In order to overcome the shortcomings of the manual procedure, several instruments utilizing mechanical or optical techniques to detect clot formation have been designed. One instrument, a clot timer manufactured by Mechrolab, uses a complex rotor that rotates or is stirred through the sample and measures resistance as coagulation occurs. A fibro-system coagulation timer by Becton, Dickinson & Co. drops a probe with fixed and movable electrodes into a cup containing blood, plasma or other material. Movable contacts in the probe cycle through the mixture. When a clot begins to form, movement of the contacts is resisted and a timer is stopped.
Instruments that combine mechanical and optical operating principles are also available. For example, some instruments employ magnetic stirrers to stir the sample and a light source and detector to detect a change in the motion of the stirrer indicative of clotting.
Seienco, Inc. of Morrison, Colo. manufactures an instrument identified as the Sonoclot coagulation analyzer. This instrument immerses a vibrating, disposable probe into a sample to measure the change in mechanical impedance imposed upon the probe by the changing visoelastic properties of the forming clot.
Each of these instruments is very expensive and bulky, thereby limiting their portability. Moreover, these instruments are extremely complex and difficult to maintain and service. In certain situations measurement of clotting times must be performed frequently and during the patient's normal workday. Consequently, it is beneficial to provide an inexpensive, small instrument to perform the desired measurements.
Research and theoretical analyses of the clotting process have also been conducted. Measurements of viscous properties in thin liquid layers close to a crystal surface have been discussed in papers by Th. Funck and F. Eggers entitled "Ultrasonic Relaxation Spectroscopy In Liquids" (appearing in Naturwissenschaften 69 1976) and "Clotting of Blood at a Gold Surface Probed by MHz Shear Quartz Resonator" (appearing in Naturwissenschaften 69 1982). These papers are devoted to the study of impedance of materials and examine clotting as a physical process. The procedures in these papers are not concerned with timing of the clotting process and employ complex instruments to perform the desired testing.